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🔗 Feature: Negotiate Authentication

🔗 Details

Negotiate is a wrapper protocol around GSSAPI, which in turn is a wrapper around either Kerberos or NTLM authentication. Why a wrapper of a wrapper? No one outside Microsoft knows at this time. GSSAPI would have been more than enough.

The real significance is that supporting it allows support of transparent Kerberos authentication to a MS Windows domain. It is significantly more secure than NTLM and also poses much less burden on the Domain Controller.

See Features/Authentication for details on other schemes supported by Squid and how authentication in general works.

🔗 How does it work in Squid?

Negotiate is at the one time both very simple and somewhat complex. Squids part has been kept intentionally minor and simple to improve the overall system security.

The authentication helper is left to perform all of the risky security encryption and validation processes. Squid may contact it initially for a challenge token and blindly relay this to the client. Any credentials or tokens presented by the client are passed untouched to the helper.

The protocol lines used to do this are described below. This same protocol is used to contact both NTLM and Negotiate authentication helpers. This allows Squid to support both Negotiate/Kerberos and Negotiate/NTLM flavours through the one protocol configuration.

:information_source: This double support does lead to some administrative confusion when the helper does not support the same flavour as the client browser.

:information_source: These authenticator schemes do not support concurrency due to the statefulness of NTLM.

🔗 Input line received from Squid:

request [credentials] [key-extras]

request is one of the request codes:

YR A new challenge token is needed. This is always the first communication between the two processes. It may also occur at any time that Squid needs a new challenge, due to the auth_param max_challenge_lifetime and max_challenge_uses parameters. The helper should respond with a TT message.
KK Authenticate a user’s credentials. The helper responds with either OK, ERR, AF, NA, or BH.

credentials is an encoded blob exactly as received in the HTTP headers. This field is only sent on KK requests.

key-extras are additional parameters passed to the helper which may be configured with auth_param key_extras parameter. Only available in Squid-3.5 and later.

🔗 Result line sent back to Squid:

result [token label] [kv-pair] [message]

result is one of the result codes:

TT Success. A new challenge token value is presented.
AF Success. Valid credentials. Deprecated by OK result from Squid-3.4 onwards.
NA Success. Invalid credentials. Deprecated by ERR result from Squid-3.4 onwards.
OK Success. Valid Credentials.
ERR Success. Invalid credentials.
BH Failure. The helper encountered a problem.

:information_source: the OK and ERR result codes are only accepted by Squid-3.4 and newer.

token is a new challenge value. The token is base64-encoded, as defined by RFC 2045.

:information_source: NOTE: NTLM authenticator interface on Squid-3.3 and older does not support a token field. Negotiate authenticator interface requires it on TT, AF and NA responses.

:information_source: This field must not be sent on OK, ERR and BH responses.

label is what gets used by Squid for this client request “username”. This field is only accepted on AF responses. It must not be sent on any other result code response.

kv-pair: one or more key=value pairs. The key names reserved on this interface:

clt_conn_tag=… Tag the client TCP connection (Squid-3.5)
group=… reserved
message=… A message string that Squid can display on an error page.
tag=… reserved
token=… The base64-encoded, as defined by RFC 2045, token to be used. This field is only used on OK responses.
ttl=… reserved
user=… The label to be used by Squid for this client request as “username”. With Negotiate and NTLM protocols it typically has the format NAME@DOMAIN or NAME\\DOMAIN respectively.
*_=… Key names ending in (_) are reserved for local administrators use.

:information_source: the kv-pair field is only accepted by Squid-3.4 and newer.

:information_source: the kv-pair returned by this helper can be logged by the %note logformat code.

message is a string that Squid can display on an error page. This field is only accepted on NA and BH responses. From Squid-3.4 this field is deprecated by the message= kv-pair on BH responses.

🔗 Squid native Windows build with NEGOTIATE support

🔗 Testing

🔗 Testing Squid

Send any URL with a GET or any other request via Squid.

squidclient http://example.com/

Squid when setup correctly replies with Proxy-Authenticate: Negotiate like shown:

HTTP/1.1 407 Proxy Authentication Required
Server: squid/
Mime-Version: 1.0
Date: Thu, 12 Jan 2012 03:41:33 GMT
Proxy-Authenticate: Negotiate

🔗 Testing the win32 helper from Unix

If you want to test the UNIX side of things out, there actually is a way to access the win32 helper from a Unix box, and that is by performing an “authorized man-in-the-middle attack”, as follows:

🔗 Troubleshooting

🔗 Token type errors

The token first presented by the client is used by helpers to identify which flavour is being used:

You may see warnings or errors mentioning either of these token types with Negotiate authentication. Particularly common are problems with type 1 when configured with Kerberos helpers.

The issue is a mismatch between the client and helper capabilities. The negotiate_wrapper helper is currently the only helper known which can handle both types at once.

Categories: Feature

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